The present invention relates to a method and apparatus for measuring the timing jitter of a tributary data stream that has been multiplexed into a higher-rate multiplex stream using pulse stuffing techniques.
Timing jitter is the short term variation of the significant instants of a data signal from their expected or average positions, the amplitude and frequency of the variation constituting the amplitude and frequency of the jitter waveform. Various ways of measuring the timing jitter of a data signal are known, one such way being described in U.S. Pat. No. 3,737,766 (Lubarsky). In this patent, the jitter amplitude of a signal is determined by comparing the actual zero crossings of the signal with their expected occurrences as predicted by an averaging process carried out on the previous actual occurrences of the zero crossings.
The present invention is concerned with the measurement of the timing jitter of a data signal where the signal has been multiplexed into a higher-rate multiplex stream using pulse stuffing techniques. Pulse stuffing is a process where the bit rate of a digital tributary stream is brought up to the rate of occurrence of time slots for that tributary in the multiplex stream by the insertion of a dummy bit or the repetition of selected tributary bits, each insertion or repetition constituting a "stuff". Each stuff may be viewed as advancing the phase of the tributary stream relative to a reference representing the average rate of occurrence of tributary time slots in the multiplex stream, the phase of the tributary thereafter falling back relative to the reference until the next stuff causes a further advance. The rate of tributary phase slippage between stuffs is, of course, dependent on the discrepancy between the tributary frequency and that of the aforesaid reference.
In practice, stuffing opportunities are only presented at fixed intervals corresponding to predetermined time slots in the multiplex stream; whether or not any particular stuffing opportunity is taken up will depend on the current phase lag of the tributary relative to the reference. Of course, in order to enable stuffed bits to be removed when demultplexing the tributary from the multiplex stream, it is necessary to provide some indication in the multiplex stream as to whether a time slot that could have been stuffed, does in fact contain a stuffed bit. This indication takes the form of stuffing control bits inserted at known locations in the multiplex stream.
Where the tributary data signal is free of jitter, it will lose phase at a constant rate dependent on the tributary frequency and this phase loss will be compensated for by stuffs at regular intervals. However, where the tributary exhibits jitter, its phase loss will not be at a constant rate with the result that stuffing will occur irregularly.